diff --git a/compiler/src/dotty/tools/dotc/transform/SyntheticMembers.scala b/compiler/src/dotty/tools/dotc/transform/SyntheticMembers.scala
index 817be9cba633..60125c5a2058 100644
--- a/compiler/src/dotty/tools/dotc/transform/SyntheticMembers.scala
+++ b/compiler/src/dotty/tools/dotc/transform/SyntheticMembers.scala
@@ -525,7 +525,7 @@ class SyntheticMembers(thisPhase: DenotTransformer) {
           val pat = Typed(untpd.Ident(nme.WILDCARD).withType(patType), TypeTree(patType))
           CaseDef(pat, EmptyTree, Literal(Constant(idx)))
         }
-      Match(param, cases)
+      Match(param.annotated(New(defn.UncheckedAnnot.typeRef, Nil)), cases)
     }
 
   /** - If `impl` is the companion of a generic sum, add `deriving.Mirror.Sum` parent
diff --git a/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala b/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala
index af02bfed8411..804a4fd33224 100644
--- a/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala
+++ b/compiler/src/dotty/tools/dotc/transform/patmat/Space.scala
@@ -503,8 +503,21 @@ class SpaceEngine(using Context) extends SpaceLogic {
     }
   }
 
+  /** Numeric literals, while being constant values of unrelated types (e.g. Char and Int),
+   *  when used in a case may end up matching at runtime, because their equals may returns true.
+   *  Because these are universally available, general purpose types, it would be good to avoid
+   *  returning false positive warnings, such as in `(c: Char) match { case 67 => ... }` emitting a
+   *  reachability warning on the case.  So the type `ConstantType(Constant(67, IntTag))` is
+   *  converted to `ConstantType(Constant(67, CharTag))`.  #12805 */
+  def convertConstantType(tp: Type, pt: Type): Type = tp match
+    case tp @ ConstantType(const) =>
+      val converted = const.convertTo(pt)
+      if converted == null then tp else ConstantType(converted)
+    case _ => tp
+
   /** Is `tp1` a subtype of `tp2`?  */
-  def isSubType(tp1: Type, tp2: Type): Boolean = {
+  def isSubType(_tp1: Type, tp2: Type): Boolean = {
+    val tp1 = convertConstantType(_tp1, tp2)
     debug.println(TypeComparer.explained(_.isSubType(tp1, tp2)))
     val res = if (ctx.explicitNulls) {
       tp1 <:< tp2
@@ -750,6 +763,7 @@ class SpaceEngine(using Context) extends SpaceLogic {
   }
 
   def show(ss: Seq[Space]): String = ss.map(show).mkString(", ")
+
   /** Display spaces */
   def show(s: Space): String = {
     def params(tp: Type): List[Type] = tp.classSymbol.primaryConstructor.info.firstParamTypes
@@ -894,13 +908,15 @@ class SpaceEngine(using Context) extends SpaceLogic {
       else
         project(OrType(selTyp, constantNullType, soft = false))
 
+    debug.println(s"targetSpace: ${show(targetSpace)}")
+
     // in redundancy check, take guard as false in order to soundly approximate
     val spaces = cases.map { x =>
       val res =
         if (x.guard.isEmpty) project(x.pat)
         else Empty
 
-      debug.println(s"${x.pat.show} ====> ${res}")
+      debug.println(s"${x.pat.show} ====> ${show(res)}")
       res
     }
 
@@ -914,12 +930,8 @@ class SpaceEngine(using Context) extends SpaceLogic {
         debug.println(s"---------------reachable? ${show(curr)}")
         debug.println(s"prev: ${show(prevs)}")
 
-        var covered = simplify(intersect(curr, targetSpace))
-        debug.println(s"covered: $covered")
-
-        // `covered == Empty` may happen for primitive types with auto-conversion
-        // see tests/patmat/reader.scala  tests/patmat/byte.scala
-        if (covered == Empty && !isNullLit(pat)) covered = curr
+        val covered = simplify(intersect(curr, targetSpace))
+        debug.println(s"covered: ${show(covered)}")
 
         if (isSubspace(covered, prevs)) {
           if i == cases.length - 1
diff --git a/tests/patmat/i12805.check b/tests/patmat/i12805.check
new file mode 100644
index 000000000000..e855c765d801
--- /dev/null
+++ b/tests/patmat/i12805.check
@@ -0,0 +1,3 @@
+10: Match case Unreachable
+16: Match case Unreachable
+22: Match case Unreachable
diff --git a/tests/patmat/i12805.scala b/tests/patmat/i12805.scala
new file mode 100644
index 000000000000..78240c2f9703
--- /dev/null
+++ b/tests/patmat/i12805.scala
@@ -0,0 +1,22 @@
+import scala.language.implicitConversions
+
+type Timeframe = "1m" | "2m" | "1H"
+type TimeframeN = 1 | 2 | 60
+
+def manualConvertToN(tf: Timeframe): TimeframeN = tf match
+  case "1m" => 1
+  case "2m" => 2
+  case "1H" => 60
+  case "4H" => ??? // was: no reachability warning
+
+given Conversion[Timeframe, TimeframeN] =
+  case "1m" => 1
+  case "2m" => 2
+  case "1H" => 60
+  case "4H" => ??? // was: no reachability warning
+
+given Conversion[TimeframeN, Timeframe] =
+  case 1 => "1m"
+  case 2 => "2m"
+  case 60 => "1H"
+  case 240 => ??? // was: no reachability warning
diff --git a/tests/patmat/i12805b.check b/tests/patmat/i12805b.check
new file mode 100644
index 000000000000..62ba8523f65e
--- /dev/null
+++ b/tests/patmat/i12805b.check
@@ -0,0 +1,3 @@
+4: Match case Unreachable
+9: Match case Unreachable
+14: Match case Unreachable
diff --git a/tests/patmat/i12805b.scala b/tests/patmat/i12805b.scala
new file mode 100644
index 000000000000..cbff6ae0d2ca
--- /dev/null
+++ b/tests/patmat/i12805b.scala
@@ -0,0 +1,14 @@
+def test1(a: 1 | 2) = a match
+  case 1 => true
+  case 2 => false
+  case 4 => ??? // unreachable case, was: no warning
+
+def test2(a: 1 | 2) = a match
+  case 1 => true
+  case 2 => false
+  case _ => ??? // unreachable
+
+def test3(a: 1 | 2) = a match
+  case 1 => true
+  case 2 => false
+  case a if a < 0 => ??? // unreachable